# Appendage displacement estimation

Discussion in 'Sailboats' started by Gog, May 11, 2021.

Tags:
1. Joined: May 2021
Posts: 2
Likes: 0, Points: 1

### GogNew Member

Hello everyone. Let me first introduce myself, I'm new to this forum and probably will stay here for a while since I'm making it to the end of my undergraduate studies in Naval Architecture at Universidad Politécnica de Madrid.

There's this project that we have to make at the end of the undergraduate and I'm struggling with the predimensioning chapter. I estimated some dimensions and I'm now designing the shapes of the canoe body (hull with no appendages) and as I want the boat to have a certain displacement, I would like to have an estimation of what fraction of the volume is actually displaced by the canoe body and how much by the appendages so as to design a coherent canoebody. Any idea on how to approach it?

2. Joined: Mar 2013
Posts: 1,110
Likes: 486, Points: 83, Legacy Rep: 39
Location: Germany

It depends on the appendage type and material, there is no hard rule. Calculations are part of the design spiral, for example you determine ballast material and amount, that gives you a volume, then you have fin and rudder area wich are usually function of total sail area, fin shape and material modifies the volume again, then you have to position the fin and ballast relative to the sails wich modifies your underbody, etc.
You start with a canoe body that represents total displacement and has "ideal" lines, then you determine rudder area and position, then you go to the (presumably) fin and bulb. Each time you compensate the underbody volume by adjusting deadrise and/or rocker, and check the result against your desired stability curve and sailing performance, plus appedages scantlings.

3. Joined: Nov 2004
Posts: 679
Likes: 106, Points: 43, Legacy Rep: 124
Location: East Anglia,England

### wet feetSenior Member

I'm a little surprised that the question is being asked by a student approaching the end of his degree studies,it seems fundamental.Is there a requirement to design the project boat the old fashioned way with pencils and paper?With a CAD design it is a matter of a few minutes work to create a ballast keel as a separate object and to determine the volume of it.Similarly the rudder(s) and the underwater portion of the canoe body.A large rudder can be an interesting thing to deal with as a hollow-or foam filled foil will have positive buoyancy and the stock and other hardware can make it close to neutral buoyancy.There is a point where chasing the last tiny amounts ceases to be useful as the weight of light switches can be obliterated by a couple of extra fenders onboard.The old approach of begin with your best estimate and refine in stages always works and the more times you have done it,the closer the first attempt is likely to be to the finished project.Along the way there may well be mistakes and overlooked features,the trend should be that the number of such mistakes will dwindle.

4. Joined: Sep 2011
Posts: 6,567
Likes: 416, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

In defense of the student who is in the last stage of his studies, and in defense of any designer with a lot of experience who asks himself the same thing (which is necessary to do in each project that is started): from what I have understood, the question is how to find out, from the total displacement of the ship, including appendages, which part is formed by the keel ballast. It is not a question of drawing a bulb and calculating its volume but of knowing, regardless of its shape, the weight it should have. It is possible, and it is necessary, to make a preliminary estimate, usually based on the observation of similar boats and much later, once the hull, sails and rigging and several other things have been defined, determine the volume, weight and its position on the boat.
This is by no means a silly question, no matter how much CAD package you have. This is how I see it and, if the student asked another question, he will clarify it for us.

5. Joined: Mar 2013
Posts: 1,110
Likes: 486, Points: 83, Legacy Rep: 39
Location: Germany

Well, that's a completely different question, and it can be approached in three different ways. First is dogmatic, set a ballast ratio you believe in, and design to it. This was common in the era of 50% ballast. Second is pragmatic, look at similar designs and start with a similar ballast ratio. Today this is around 30%. Third is "scientific", ballast is a function of stability, and depends on sail area, draft, and form stability. A good example for this are production boats that have identical hulls but different keels (for draft reasons) and rigs (racer/cruiser).

6. Joined: Aug 2004
Posts: 2,982
Likes: 506, Points: 113, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

Actually it is fairly easy if a bit fiddley. Realistically, this should be done each time through the design spiral and no latter than the second time through using "peanut butter" structural weights.

7. Joined: May 2021
Posts: 2
Likes: 0, Points: 1

### GogNew Member

Thank you so much for all the answers, but I think the question was misunderstood when, in my opinion, was pretty specific.

I'm not talking about ballast weight, I just want to have a rule of thumb of what fraction of the displaced VOLUME will be displaced by appendages without going through a completely different design process from the one that I already planned following.

I know that there are a lot of types of appendages. For more detail: I'll be designing a fin keel with spade rudder in a 17 tons sloop with a max draught of 2,6 meters.

8. Joined: Aug 2004
Posts: 2,982
Likes: 506, Points: 113, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

Oh, I think most of us understood the very non-specific question you asked. The answer to the question you think you asked ("I would like to have an estimation of what fraction of the volume is actually displaced by the canoe body and how much by the appendages so as to design a coherent canoebody") is that appendages are somewhere between ~2% to ~35+% of the of the total volume depending on the canoe body hull form and other design parameters. In regard to the question you actually asked (Any idea on how to approach it?), I gave you the methodology.
Finally; if you actually need this before you can begin the design process you planned, you have already set yourself up for failure and my advice would be to re-assess your design methodology. But, to humor you, I know the number you are looking for is that modern fin keel appendages are between 10-15% of the total displacement. If you want a precise (not accurate) percentage, I would suggest you go find 5-6 sailboats with a look similar to what you want and then average their appendage volume being careful to separate out what is a function of the appendage root and what is part of the canoe hull.
Good luck...

Rumars likes this.

9. Joined: Aug 2002
Posts: 15,198
Likes: 928, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

Gog: Can you clarify your question? The answer depends on whether you need to learn how to measure the volume of an irregular body, or you need to know the ratio of volumes between appendages and hull. The first is pretty straightforward. The second is the annoying "depends". It depends on the design and has to fulfill the SOR. However, looking at successful designs and the ratio/shapes they have is a good way of judging your design.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.